LED including photonic crystal structure

ABSTRACT

A photonic crystal light emitting diode (“PXLED”) is provided. The PXLED includes a periodic structure, such as a lattice of holes, formed in the semiconductor layers of an LED. The parameters of the periodic structure are such that the energy of the photons, emitted by the PXLED, lies close to a band edge of the band structure of the periodic structure. Metal electrode layers have a strong influence on the efficiency of the PXLEDs. Also, PXLEDs formed from GaN have a low surface recombination velocity and hence a high efficiency. The PXLEDs are formed with novel fabrication techniques, such as the epitaxial lateral overgrowth technique over a patterned masking layer, yielding semiconductor layers with low defect density. Inverting the PXLED to expose the pattern of the masking layer or using the Talbot effect to create an aligned second patterned masking layer allows the formation of PXLEDs with low defect density.

BACKGROUND

1. Field of Invention

The present invention relates to light emitting diodes, moreparticularly to light emitting diodes with a photonic crystal structure.

2. Description of Related Art

Light emitting diodes (“LEDs”) are technologically and economicallyadvantageous solid state light sources. LEDs are capable of reliablyproviding light with high brightness, hence in the past decades theyhave come to play a critical role in numerous applications, includingflat-panel displays, traffic lights, and optical communications. An LEDincludes a forward biased p-n junction. When driven by a current,electrons and holes are injected into the junction region, where theyrecombine, releasing their energy by emitting photons. The quality of anLED can be characterized, for example, by its extraction efficiency thatmeasures the intensity of the emitted light for a given number ofphotons generated within the LED chip. The extraction efficiency islimited, among others, by the emitted photons suffering multiple totalinternal reflections at the walls of the high refractive indexsemiconductor medium. As a result, the emitted photons do not escapeinto free space, leading to poor extraction efficiencies, typically lessthan 30%.

In the past thirty years, various approaches have been proposed toenhance the extraction efficiency of LEDs. The extraction efficiency canbe increased, for example, by enlarging the spatial angle in which theemitted photons can escape by developing suitable geometries, includingcubic, cylindrical, pyramidal, and dome like shapes. However, none ofthese geometries can entirely eliminate losses from total reflection.

A further source of loss is the reflection caused by the refractiveindex mismatch between the LED and the surrounding media. While suchlosses could be reduced with an anti-reflection coating, completecancellation of reflection can be achieved only at a specific photonenergy and one angle of incidence.

U.S. Pat. No. 5,955,749, entitled “Light Emitting Device Utilizing aPeriodic Dielectric Structure,” granted to J. Joannopoulos et al.,describes an approach to the problem of enhancing the extractionefficiency. According to U.S. Pat. No. 5,955,749 a photonic crystal iscreated by forming a lattice of holes in the semiconductor layers of thelight emitting diode. The lattice of holes creates a medium with aperiodically modulated dielectric constant, affecting the way lightpropagates through the medium. The photons of the light emitting diodecan be characterized by their spectrum or dispersion relation,describing the relation between the energy and the wavelength of thephotons. The spectrum of a photonic crystal consists of two classes.Photons in the radiative class have energies and wavelengths that matchthe spectrum of photons in free space thus the radiative photons arecapable of escaping from the light emitting diode. Photons in the guidedclass, on the other hand, have energies and wavelengths that do notmatch the spectrum of photons in free space; therefore, guided photonsare trapped in the light emitting diode. The guided photons areanalogous to the earlier described photons, suffering total internalreflections.

The spectrum of guided photons in the photonic crystal consists ofenergy bands, or photonic bands, separated by band gaps, in analogy withthe spectrum of electrons in crystalline lattices. Guided photons withenergies in the band gap cannot propagate in the photonic crystal. Incontrast, the spectrum of the radiative photons is a continuum, and thushas no gap. The recombinative processes in a typical LED emit photonswith a well-defined energy. If, therefore, a photonic crystal is formedin the LED such that the energy of the emitted photons falls within theband gap of the photonic crystal, then all the emitted photons areemitted as radiative photons as no guided photons can exist with suchenergies. As described above, since all the radiative photons arecapable of escaping from the LED, this design increases the extractionefficiency of the LED.

In an effort to explore the usefulness of photonic crystals for lightgeneration, U.S. Pat. No. 5,955,749 gives a partial description of atheoretical structure of a photonic crystal device.

U.S. Pat. No. 5,955,749 describes an n-doped layer, an active layer, anda p-doped layer, and a lattice of holes formed in these layers. However,the device of U.S. Pat. No. 5,955,749 is not operational and thereforeis not a LED. First, electrodes are not described, even though those areneeded for the successful operation of a photonic crystal LED (“PXLED”).The fabrication of electrodes in regular LEDs is known in the art.However, for PXLEDs, neither the fabrication of electrodes, nor theirinfluence on the operation of the PXLED is obvious. For example,suitably aligning the mask of the electrode layer with the lattice ofholes may require new fabrication techniques. Also, electrodes aretypically thought to reduce the extraction efficiency as they reflect aportion of the emitted photons back into the LED, and absorb anotherportion of the emitted light.

Second, U.S. Pat. No. 5,955,749 proposes fabricating photonic crystallight emitting devices from GaAs. GaAs is indeed a convenient and hencepopular material to fabricate regular LEDs. However, it has a high“surface recombination velocity” of about 10⁶ cm/sec as described, forexample, by S. Tiwari in “Compound Semiconductor Devices Physics,”Academic Press (1992). The surface recombination velocity expresses therate of the recombination of electrons and holes on the surface of thediode. Electrons and holes are present in the junction region of theLED, coming from the n-doped layer and the p-doped layer, respectively.When electrons and holes recombine across the semiconductor gap, therecombination energy is emitted in the form of photons and generateslight. However, when electrons and holes recombine through intermediateelectronic states in the gap, then the recombination energy is emittedin the form of heat instead of photons, reducing the light emissionefficiency of the LED. In an ideal crystal there are no states in thegap. Also, in today's high purity semiconductor crystals there are veryfew states in the gap in the bulk material. However, on the surface ofsemiconductors typically there are a large number of surface states anddefect states, many of them in the gap. Therefore, a large fraction ofelectrons and holes that are close to the surface will recombine throughthese surface and defect states. This surface recombination generatesheat instead of light, considerably reducing the efficiency of the LED.

This problem does not result in a serious loss of efficiency for regularLED structures. However, PXLEDs include a large number of holes, thusPXLEDs have a much larger surface area than the regular LEDs. Therefore,the surface recombination may be capable of reducing the efficiency ofthe PXLED below the efficiency of the same LED without the photoniccrystal structure, making the formation of photonic crystal structurepointless. Since GaAs has a high surface recombination velocity, it isnot a promising candidate for fabricating photonic crystal LEDs. Theseriousness of the problem is reflected by the fact that so far, toApplicants' knowledge, no operating LED with a photonic crystal near theactive region has been reported in the literature that uses GaAs andclaims an enhanced extraction, or internal, efficiency. In particular,U.S. Pat. No. 5,955,749 does not describe the successful operation of aphotonic crystal LED. Also, U.S. Pat. No. 5,955,749 does not describethe influence of the photonic crystal on the emission process, which canaffect the internal efficiency of the LED.

While photonic crystals are promising for light extraction for thereasons described above, there are problems with the design. There areseveral publications describing experiments on a lattice of holes havingbeen formed in a slab of a semiconductor. An enhancement of theextraction rate at photon energies in the bandgap has been reported byR. K. Lee et al. in “Modified Spontaneous Emission From aTwo-dimensional Photonic Bandgap Crystal Slab,” in the Journal of theOptical Society of America B, vol. 17, page 1438 (2000). Lee et al. notonly shows the extraction benefits of a photonic crystal in a lightemitting design, but also shows that the photonic lattice can influencethe spontaneous emission. However, Lee et al. do not show how to formand operate a light emitting device with this design. A photonic crystalLED can be formed from Lee et al.'s light emitting design by includingelectrodes. The addition of the electrodes, however, will substantiallyaffect the extraction and the spontaneous emission. Since this effect isunknown, it cannot be disregarded in the design of a LED. Since the Leeet al. design does not include such electrodes, the overallcharacteristics of an LED, formed from that design, are unclear. Thisquestions the usefulness of the design of Lee et al.

Therefore, there is a need for new designs to create operationalphotonic crystal LEDs. This need includes the introduction of newmaterials that have sufficiently low surface recombination velocities.The need also extends to designs that counteract predicted negativeeffects, such as reduced spontaneous emission rates and reflection byelectrodes. Finally, there is a need for describing techniques for thefabrication of photonic crystal LEDs, including fabricating electrodes.

SUMMARY

According to the invention a photonic crystal light emitting diode isprovided. The PXLED includes an n-doped layer, a light emitting activelayer, a p-doped layer, and electrodes for the n-doped and p-dopedlayers. A photonic crystal is formed as a periodic structure in theactive layer, or in one of the doped layers, extending distances thatare close to, or through, the active layer. In one embodiment theperiodic structure is a two dimensional lattice of holes. The holes canhave circular, square or hexagonal cross sections. The holes can befilled with air or with a dielectric. In another embodiment the periodicstructure is periodic in only one dimension, an example of which is aset of parallel grooves. In another embodiment the dielectric constantof the PXLED can vary in one or two directions within the plane of thesemiconductor layers. In another embodiment the thickness of theselected layers can vary in one or two directions within the plane ofthe semiconductor layers.

The parameters characterizing the lattice of the holes, include thelattice constant, the diameter of the holes, the depth of the holes, andthe dielectric constant of the dielectric in the holes. In someembodiments these parameters are chosen such that the wavelength of theemitted light lies close to the edge of the energy bands of the photoniccrystal, because close to the band edge the density of states of thephotons is large. The recombination energy can be emitted much moreefficiently through photons with a large density of states. Therefore,in embodiments of the present invention that emit light with energiesclose to the band edge, the emitted power can exceed the power emittedby the same LED without the periodic structure up to about eight times.This enhancement can be related to the presence of metal electrodelayers in embodiments of the invention that enhance the efficiency andincrease the emitted power of the PXLEDs.

The present embodiments are formed from III-Nitride compounds, whichinclude Nitrogen and a group III element, such as Gallium, Aluminum, orIndium. III-Nitride compounds are used because their surfacerecombination velocities are more than ten times smaller than that ofGaAs, according to M. Boroditsky et al., in J. App. Phys. vol. 87, p.3497 (2000). As described above, a low surface recombination velocitycan increase the efficiency of a PXLED above the efficiency of a regularLED without the photonic crystal structure, making GaN PXLEDstechnically and economically viable candidates for improved lightgeneration efficiency.

Additionally, GaN LEDs are the leading candidates for generating lightin the blue and green regime of the spectrum; therefore, increasingtheir efficiency is highly desired. Finally, since the external quantumefficiency of GaN LEDs is often in the vicinity of 10 percent, theformation of photonic crystals can improve the efficiency of a GaN LEDin a substantial manner. Here the external quantum efficiency is theproduct of the internal quantum efficiency and the extractionefficiency.

The new structure of PXLEDs uses novel fabrication techniques. Somemethods of the invention create a PXLED by forming an n-doped layer, anactive layer overlying the n-doped layer, a p-doped layer overlying theactive layer, and a p-electrode layer overlying the p-doped layer. Insome embodiments, the n-doped layer, the active layer, and the p-dopedlayer can include one or more layers. Next, a patterned masking layer isformed with openings, overlying the p-doped layer. Through the openingsof the masking layer the p-electrode layer and the underlyingsemiconductor layers are removed to form a lattice of holes withsuitably chosen cross sections. Finally, the masking layer is removed,and an n-electrode layer is deposited on the n-doped layer.

Some methods of the invention create a PXLED by forming a patternedmasking layer with openings on a substrate. Then the epitaxial lateralovergrowth technique (“ELOG”) is used to form an n-doped layer overlyingthe masking layer, an active layer overlying the n-doped layer, ap-doped layer overlying the active layer, and a p-electrode layer on thep-doped layer. The ELOG technique creates semiconductor layers with alow density of defects, improving the performance and reliability of thePXLEDs. A second substrate is formed on the electrode layer and thefirst substrate is removed to expose the masking layer. Next, thesemiconductor layers are at least partially removed through the openingsof the masking layer to form a lattice of holes. Finally, the maskinglayer is used as the p-electrode layer, and an n-electrode layer isformed on the n-doped layer.

Some methods of the invention create a PXLED by forming a first maskinglayer on a substrate. Then the epitaxial lateral overgrowth technique isused to form an n-doped layer overlying the masking layer, an activelayer overlying the n-doped layer, and a p-doped layer overlying theactive layer. Next, the Talbot effect is used to form a second patternedmasking layer overlying the p-doped layer, utilizing the diffraction oflight across the openings of the first masking layer. Next, thesemiconductor layers are at least partially removed through the openingsof the first masking layer to form a lattice of holes. Finally,electrode layers are formed for both the n-doped layer and the p-dopedlayer.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a side view of an embodiment of a photonic crystallight emitting diode.

FIG. 2 illustrates a side view of another embodiment of a photoniccrystal light emitting diode.

FIG. 3 illustrates a top view of an embodiment of a photonic crystallight emitting diode.

FIG. 4 illustrates the relative emission (solid line) and extractionefficiency (dashed line) for the embodiment of FIG. 1 as a function of anormalized frequency.

FIG. 5A illustrates the relative emission (solid line) and extractionefficiency (dashed line) for the embodiment of FIG. 2 as a function of anormalized frequency.

FIG. 5B illustrates the product of the relative emission and theextraction efficiency as a function of a normalized frequency.

FIGS. 6A-E illustrate steps of a method for making a photonic crystallight emitting diode.

FIGS. 7A-F illustrate steps of another method for making a photoniccrystal light emitting diode.

FIGS. 8A-G illustrate steps of another method for making a photoniccrystal light emitting diode.

FIGS. 9A-F illustrate steps of another method for making a photoniccrystal light emitting diode.

FIGS. 10A-E illustrate steps of another method for making a photoniccrystal light emitting diode.

FIGS. 11A-E illustrate steps of another method for making a photoniccrystal light emitting diode.

FIGS. 12A-E illustrate steps of another method for making a photoniccrystal light emitting diode.

FIG. 13 illustrates a packaged LED.

DETAILED DESCRIPTION

FIG. 1 illustrates an embodiment of a photonic crystal LED (“PXLED”)100. A first electrode layer 104 is formed from a thick andsubstantially reflective metal. In some embodiments first electrodelayer 104 also serves as a substrate. In some embodiments firstelectrode layer 104 can overlie a substrate. Many different metals canbe used for forming first electrode layer 104, including Au, Al, Ag, andheavily doped semiconductors. An n-doped layer 108 overlies firstelectrode layer 104. An active layer 112 overlies n-doped layer 108. Ap-doped layer 116 overlies active layer 112. Finally, a second electrodelayer 120 overlies p-doped layer 116. Semiconductor layers 108, 112, and116 are often referred to as epi-layers 124. Throughout the presentapplication the term “layer” can refer to either a single semiconductorlayer or a multi-layer structure, where the individual layers of themulti-layer differ in dopant concentration, alloy composition, or someother physical characteristics.

Active layer 112 includes the junction region where the electrons ofn-doped layer 108 recombine with the holes of p-doped layer 116 and emitthe energy of recombination in the form of photons. Active layer 112 mayinclude a quantum well structure to optimize the generation of photons.Many different quantum well structures have been described in theliterature, for example, by G. B. Stringfellow and M. George Crawford in“High Brightness Light Emitting Diodes,” published by the AssociatedPress in 1997.

The photonic crystal of the PXLED is created by forming a periodicstructure in the LED. The periodic structure can include a periodicvariation of the thickness of p-doped layer 116, with alternating maximaand minima. An example is a planar lattice of holes 122-i, where theinteger i indexes the holes. In the present embodiment holes 122-i arethrough holes, formed in n-doped layer 108, in active layer 112, and inp-doped layer 116. In some embodiments holes 122-i are formed in p-dopedlayer 116 and active layer 112. In some embodiments holes 122-i areformed only in p-doped layer 116, extending to the proximity of activelayer 112. For example, holes 122-i can extend to within one wavelengthof the emitted light—in the p-doped layer 116—from active layer 112. Insome embodiments the ratio of the period of the periodic structure andthe wavelength of the emitted light—in air—lies in the range of about0.1 to about 5. In embodiments, with period—to—wavelength ratios in the0.1 to 5 range, the formation of the photonic crystal may significantlyinfluence the efficiency of PXLED 100.

Holes 122-i can have circular, square, hexagonal, and several othertypes of cross sections. Also, holes 122-i can be filled with air orwith a dielectric of dielectric constant ∈_(h), differing from thedielectric constant of epi-layers 124. Possible dielectrics includesilicon oxide.

FIG. 2 illustrates another embodiment of the invention. PXLED 100 isformed on host substrate 102, n-doped layer 108 overlying host substrate102, active layer 112 overlying n-doped layer 108, p-doped layer 116overlying active layer 112, and second electrode layer 120 overlyingp-doped layer 116. In this embodiment n-electrode layer 104 is formedoverlying an area of n-doped layer 108 away from the photonic crystal,making the fabrication of n-electrode layer 104 technologically simple.The fabrication of such embodiments is described, for example, in U.S.Pat. No. 6,307,218 B1, “Electrode Structures of Light Emitting Devices,”by D. Steigerwald et al., hereby incorporated in its entirety by thisreference.

FIG. 3 illustrates a photonic crystal structure, formed as a triangularlattice of holes 122-i. The lattice is characterized by the diameter ofholes 122-i, d, the lattice constant a, which measures the distancebetween the centers of nearest neighbor holes, the depth of the holes w(shown e.g. in FIG. 1), and the dielectric constant of the dielectric,disposed in the holes, ∈_(h). The PXLED parameters a, d, w, and ∈_(h)influence the density of states of the bands, and in particular, thedensity of states at the band edges of the photonic crystal's spectrum.

The lattice structure of the lattice of holes 122-i also influences theefficiency. In various embodiments, holes 122-i form square, hexagonal,honeycomb, and other well-known two-dimensional lattices.

FIG. 4 illustrates the efficiency of a particular embodiment of FIG. 1.While the efficiency indicators will be described in relation to aparticular embodiment of FIG. 1, in analogous embodiments the efficiencyindicators demonstrate analogous behavior. In this particular embodimentof FIG. 1 epi-layers 124 are made from AlGaInN, with a suitably chosenalloying stoichiometry of the elements Al, Ga, In, and N. N-doped layer108 is doped with silicon and p-doped layer 116 is doped with magnesium.N-doped layer 108 and p-doped layer 116 are designed for efficientcarrier injection into active layer 112. Active layer 112 includes InGaNlayers, forming quantum wells, sandwiched between n-type InGaN layerswith lower In concentration. The wavelength λ of the emitted light canbe tuned by suitably choosing the In concentration and thickness of thequantum wells. First and second electrode layers 104 and 120 are formedof highly reflective and low loss materials, including Ag, Al, and Aubased electrode materials. The lattice is a triangular lattice as shownin FIG. 3. The total thickness of epi layers 124 are between 0.375a and2a and the hole diameter d is 0.72a. The first electrode 104 has athickness of a, or greater, and the second electrode 120 has a thicknessof 0.03a. The location of the active layer 112 is 0.0625a away from thecenter of epi-layers 124, closer to first electrode layer 104. Theefficiency of PXLED 100 is sensitive to the location of active layer112.

FIG. 4 illustrates two indicators of the efficiency of the aboveparticular embodiment of FIG. 1. The solid line indicates the relativeemission, while the dashed line indicates the extraction efficiency. Therelative emission is defined as the ratio of the total power emitted inthe form of light by an LED with a photonic crystal structure, dividedby the total power emitted by the same LED, but without the periodicstructure. The efficiency indicators are shown in FIG. 4 as a functionof the photon frequency ν, normalized by c/a, wherein c is the speed oflight—in air—and a is the lattice spacing. The photon frequency ν andthe photon energy E are related through the well-known relation: E=hν,where h is Planck's constant. As illustrated in FIG. 4, the relativeemission shows a maximum in the vicinity of the value ν/(c/a)=0.70. Therelative emission of PXLED 100 is approximately eight times larger atthe maximum compared to the relative emission of the same LED, butwithout the photonic crystal structure. The extraction efficiency ofPXLED 100 is relatively flat as a function of the frequency. The totalefficiency of PXLED 100 is proportional to the product of thesequantities, clearly showing that forming a photonic crystal in an LEDenhances the total efficiency and thus the emitted power, if theparameters of PXLED 100 are chosen suitably.

In this embodiment the lattice is designed so that a maximum of therelative emission, and thus the total efficiency, occurs at or near thefrequency of the emitted light. In the embodiment of FIG. 4 the maximumof the relative emission occurs around the frequency of ν=0.7(c/a).Therefore, in a PXLED where the active layer emits light with wavelengthλ, a local maximum of the relative emission will substantially coincidewith the frequency of the emitted light, if the lattice spacing of thephotonic crystal a is about 0.7λ. Here the relation λν=c has been usedto connect the wavelength λ and the frequency ν. For example, if thewavelength of the emitted light λ is 530 nm, then the lattice spacing ais 371 nm.

The analysis of the band structure and the corresponding density ofstates reveal that the above enhancement of the power occurs at energiesclose to the band edge. The density of the photons is large close to theband edge. The rate of spontaneous emission is proportional to thedensity of states. Thus, a large density of states enhances the rate ofspontaneous emission. Therefore, embodiments are designed so that theenergy of the emitted light lies close to the band edge, thus enhancingthe efficiency of the PXLED. Furthermore, the PXLED parameters a, d, w,and ∈_(h), and the design of the electrode layers can be selected toenhance the extraction efficiency as well, maximizing the totalefficiency of the PXLED. The efficiency of the PXLED shows significantsensitivity to the presence and design of the electrode layers.

In other embodiments typical values of the lattice spacing a lie betweenabout 0.1 λ and about 10 λ, preferably between about 0.1 λ and about 4λ. Typical values for the hole diameter d lie between about 0.1 a andabout 0.5 a. Typical values of the depth of the hole w lie between zeroand the fill thickness of epi-layers 124. Finally, ∈_(h) typically liesbetween 1 and about 16.

In embodiments, where epi-layers 124 have a total thickness of about 2aor higher, the maximum value of the relative emission is 2-3 foldsmaller. In these embodiments the maximum value is less sensitive to thepresence or absence of electrode layers 104 and 120.

In embodiments, where the electrode layers have substantial dissipativeproperties, such as a substantial imaginary part of the refractive indexn, the design parameters may differ considerably from theabove-described values. The efficiency of the emission depends on thepolarization of the emitted light. However, PXLEDs with a honeycomblattice of holes are capable of emitting light with an emissionefficiency that is polarization insensitive.

FIG. 5A shows the two indicators of the efficiency of the embodiment ofFIG. 2. While the efficiency indicators will be described in relation toa particular embodiment of FIG. 2, in analogous embodiments theefficiency indicators demonstrate analogous behavior. In the embodimentof FIG. 2, epi-layers 124 are only partially removed in holes 122-i. Thedepth of holes 122-i is 2a in this particular embodiment. In thisparticular embodiment of FIG. 3, the total thickness of epi-layers 124is greater than about 2a, for example 6a. The air-filled holes have adiameter d of about 0.72 a, and the depth of the holes is about 2a.Similar results hold for hole diameters lying in the range of about 0.3a and about a. Second electrode layer 120 has a thickness of about 0.09a, and active layer 112 is formed with a thickness of about 0.5 a.

FIG. 5A illustrates two indicators of the efficiency of the aboveparticular embodiment of FIG. 2. Again, the solid line indicates therelative emission, while the dashed line indicates the extractionefficiency. In analogy with FIG. 4, the relative emission shows anenhancement with a maximum of about 2.7 at a frequency of aboutν=0.325(c/a). Furthermore, in this embodiment the extraction rate alsovaries as a function of the frequency, unlike in FIG. 4. In particular,the extraction rate exhibits broad maxima in the 0.3(c/a) to 0.45(c/a)range, and around 0.65(c/a).

FIG. 5B illustrates the product of the extraction efficiency and therelative emission, the two quantities shown in FIG. 5A. As mentionedbefore, the total efficiency of the PXLED is proportional to thisproduct. As demonstrated by FIG. 5B, the emitted power of thisembodiment is once again greater than that of the corresponding LEDwithout the photonic crystal structure.

According to FIG. 5B, the total efficiency shows maxima at thenormalized frequencies of about ν=0.325(c/a) and about ν=0.63(c/a).Therefore, in a PXLED where the active layer emits light with wavelengthλ, local maxima of the relative emission will substantially coincidewith the frequency of the emitted light, if the lattice spacing of thephotonic crystal a is about a=0.325λ or about a=0.63λ. Here again therelation λν=c has been used to connect the wavelength λ and thefrequency ν. For example if the emitted wavelength λ is 530 nm then thelattice spacing a can be suitably chosen to be about 172 nm or about 334nm.

Some embodiments show a resonant behavior at some frequencies. At theseresonant frequencies the pattern of emission can be different from theemission at other frequencies. For example, in the vicinity of thefrequency ν/(c/a)=0.54 the embodiment of FIG. 2 radiates its powermostly towards the second electrode layer instead of the first electrodelayer, resulting in a minimum of the extraction efficiency. Theexistence of this minimum once again underscores the importance of theelectrode layers. This effect can be used to design PXLEDs that emit alarge fraction of the generated light into a selected direction.

In embodiments, where the electrode layers 104 and 120 have substantialdissipative properties, such as a substantial imaginary part of therefractive index n, the design parameters may differ considerably fromthe above-described values.

The periodic structure can be made three-dimensional by creating avariation of the dielectric constant of one or more selectedsemiconductor layers in the direction normal to the plane of the layersbesides the already formed two-dimensional periodic structure. This canbe achieved, for example, by forming several structural layers within aselected semiconductor layer, the structural layers having two differentalloy compositions in an alternating manner.

In some embodiments the periodic structure is a variation of thethickness of one or more selected semiconductor layers. The periodicstructure can include variations of the thickness along one directionwithin the plane of the semiconductor layers, but extending along asecond direction without variation, in essence forming a set of parallelgrooves. Two-dimensional periodic variations of the thickness includevarious lattices of indentations.

While the present embodiment and further embodiments below are describedhaving an n-doped layer deposited first and a p-doped layer formedoverlying the n-doped layer, LEDs with the opposite architecture, wherea p-doped layer is deposited first and an n-doped layer is formedoverlying the p-doped layer, are also understood to be within the scopeof the invention.

As explained above, semiconductors with low surface recombinationvelocities are promising candidates for forming PXLEDs. Electrons andholes recombining on the surface via mid-gap states release their energyin the form of heat instead of light. Therefore, the surface acts as acurrent sink, reducing the efficiency of PXLEDs. The reduction ofefficiency is high in PXLEDS formed from semiconductors with highsurface recombination velocities, such as GaAs. In fact, the efficiencyof GaAs PXLEDs can be reduced below the efficiency of GaAs LEDs with thesame architecture, but without the photonic crystal structure. For thisreason, fabricating PXLEDs from GaAs does not offer significantadvantages.

In contrast, forming a photonic crystal structure in GaN LEDs cansignificantly increase the efficiency of the GaN LEDs, because GaN has amuch lower surface recombination velocity than GaAs.

Therefore, in embodiments of the present invention epi-layers 124 areformed from semiconductors with low surface recombination velocities.Suitable choices include III-Nitride semiconductors, formed fromNitrogen and a group III element, such as Gallium. The advantages ofthis choice can be appreciated by noting that the surface recombinationvelocity of GaAs is about 10⁶ cm/sec, whereas the surface recombinationvelocity of GaN is about 3×10⁴ cm/sec. The low surface recombinationvelocity makes the surface recombination process much weaker in GaN thanin GaAs. Furthermore, the diffusion length of carriers in GaN is alsomuch smaller than in GaAs. Therefore, much fewer carriers diffuse ontoto the surface in GaN than in GaAs during traversing the LED. Thesmallness of the number of carriers, reaching the surface by diffusion,further weakens the already weak surface recombination process.

III-Nitride LEDs can also be formed using AlGaN, InGaN or combinationsthereof.

The novel structure of the PXLEDs can be fabricated in novel ways. FIGS.6A-D illustrate a method of fabricating PXLEDs.

FIG. 6A illustrates the step of forming PXLED 100 on host substrate 102,which can be, for example, sapphire. N-doped layer 108, active layer112, p-doped layer 116, and second electrode layer 120 are formed byusual deposition techniques. Masking layer 128 is formed overlyingsecond electrode layer 120.

FIG. 6B illustrates the step of patterning lattice of openings 130-iinto masking layer 128 using a high resolution lithography technique,such as electron beam lithography, nano-imprint lithography, deep X-raylithography, interferometric lithography, hot embossing or microcontactprinting.

FIG. 6C illustrates the step of at least partially removing epi-layers124 corresponding to lattice of openings 130-i of masking layer 128. InFIG. 6C n-doped layer 108 is removed only partially. Approximatelyvertical walls can be achieved by using dry etching techniques. Thedamage caused by a dry etching can be reduced by a subsequent short wetchemical etching, annealing, a combination thereof or other surfacepassivation techniques.

FIG. 6D illustrates the step of removing masking layer 128. This stepexposes second electrode layer 120 that can hence be used to provideelectrical contact to p-doped layer 116. Finally, first electrode layer104 is formed on a region of n-doped layer 108, from where p-doped layer116 and active layer 112 have been removed. In some embodiments n-dopedlayer 108 has been partially removed as well in that region. Firstelectrode layer 104 can be formed in a region of n-doped layer 108 thatis displaced from the photonic crystal structure, making its fabricationeasier. Lateral compact geometries for the formation of first electrodelayer 104 have been described in U.S. Pat. No. 6,307,218 B1, “ElectrodeStructures of Light Emitting Devices,” by D. Steigerwald et al.

In LEDs the currents flow between first electrode layer 104 and secondelectrode layer 120. Since in the above-described embodiments firstelectrode layer 104 and second electrode layer 120 are formed athorizontally removed areas, the flow of the currents includessubstantially horizontal pathways.

In some embodiments host substrate 102 is a good conductor, thus firstelectrode layer 104 can be deposited on host substrate 102 directly. Inthese embodiments the pathways of the currents are substantiallyvertical across epi-layers 124.

Some embodiments emit most of the generated light through host substrate102, while other embodiments emit most of the light through the sideopposite to host substrate 102, sometimes referred to as the top of theLED. In substrate-emitting PXLEDs host substrate 102 is formed from asubstantially transparent material and second electrode layer 120 isformed from a substantially reflective or opaque material. Intop-emitting PXLEDs host substrate 102 is formed from a substantiallyreflective or opaque material. In some embodiments a reflective layer isdeposited on host substrate 102.

FIG. 6E illustrates that in some embodiments where host substrate 102 isconductive first electrode layer 104 can be formed on the side of hostsubstrate 102 opposite of epi-layers 124. In these embodiments thecurrent pathways are substantially vertical across the entire PXLED 100.

FIGS. 7A-F illustrate another method of fabricating PXLEDs. Thisepitaxial lateral overgrowth (“ELOG”) technique can be useful, forexample, for III-Nitride based semiconductor structures, such asGaN-based LEDs. GaN semiconductors have an unusually large concentrationof defects, including fractures and dislocations. This high defectconcentration can lead to poor reliability, reduced efficiency, anddiminished brightness. Many of the defects are nucleated by the surfaceof the growth substrate. The ELOG technique reduces the defectconcentration, significantly reducing the above detrimental effects.

FIG. 7A illustrates the step of forming masking layer 128 on firstsubstrate 102. Lattice of openings 130-i can be formed in masking layer128 by high resolution lithographic techniques, such as electron beamlithography, nano-imprint lithography, deep X-ray lithography,interferometric lithography, hot embossing or microcontact printing.

FIG. 7B illustrates the step of forming n-doped layer 108 overlyingfirst substrate 102 and masking layer 128. Active layer 112 is formedoverlying n-doped layer 108, and p-doped layer 116 is formed overlyingactive layer 112. A feature of the ELOG technique is that n-doped layer108 primarily grows starting from first substrate 102 through lattice ofopenings 130-i. Thus, the growing n-doped layer 108 spreads outlaterally into regions 138-i, rather than grow straight up from maskinglayer 128.

The defects are typically nucleated by first substrate 102, and hencewill originate primarily in lattice of openings 130-i. As the growth ofn-doped layer 108 spreads out into regions 138-i, defects anddislocations tend to turn out and annihilate one another in theovergrown region. Therefore, the defect concentration will be high indefect-rich regions 134-i directly above lattice of openings 130-i,whereas the defect concentration will be low in defect-poor regions138-i, between lattice of openings 130-i.

FIG. 7C illustrates the step of forming bonding layer 121 and secondsubstrate 142 overlying p-doped layer 116. Bonding layer 121 bondsepi-layers 124 to second substrate 142.

FIG. 7D illustrates the step of removing first substrate 102 fromepi-layers 124 using laser lift-off, or etching techniques.

FIG. 7E illustrates the step of using masking layer 128 to form holes122-i by an etching procedure. For example, dry etching can be used toensure that the walls of holes 122-i are approximately vertical. Theopenings 130-i of masking layer 128 are aligned with defect-rich regions134-i. Therefore, the etching step removes the regions with high defectdensity. Therefore, only the epi-layers 124 that were formed in thedefect-poor regions 138-i are left by this etching step, yielding aPXLED 100 with low defect density and thus high quality.

FIG. 7F illustrates the step of removing masking layer 128, and formingfirst electrode layer 104 on top of defect-poor regions 138-i. Firstelectrode layer 104 can be formed, for example, by deposition from anangle. This technique minimizes the deposition of contact materialsinside holes 122-i. Second electrode layer 120 can be formed athorizontally removed areas.

In some embodiments masking layer 128 itself can serve as firstelectrode layer 104. In these embodiments masking layer 128 is notremoved.

In substrate-emitting PXLEDs bonding layer 121 is substantiallytransparent, formed from, for example, indium tin oxide (“ITO”). Secondsubstrate 142 is also substantially transparent, formed from, forexample, sapphire, silicon carbide or glass. First electrode layer 104is substantially reflective or opaque, formed from, for example, Ag, Alor Au.

In top-emitting PXLEDs, at least one of bonding layer 121 and secondsubstrate 142 are substantially reflective or opaque. Bonding layer 121or second substrate 142 can be made reflective, for example, by forminga substantially reflective overlying layer.

FIGS. 8A-G illustrate a related method of fabricating PXLEDs. The stepsshown in FIGS. 8A-D are the same as in FIGS. 7A-D.

FIG. 8E illustrates the step of forming photosensitive layer 148. Thetransparency of masking layer 128 is low. To capitalize on thisproperty, a negative photosensitive layer 148 is deposited over thesurface from where first substrate 102 has been removed. Negativephotosensitive layer 148 is deposited over masking layer 128 anddefect-rich regions 134-i. Next, light is shone through second substrate142, reaching photosensitive layer 148 across epi-layers 124. Negativephotosensitive layer 148 changes its chemical composition where it isexposed to the incident light. This change in chemical composition makesit possible to remove negative photosensitive layer 148 overlyingmasking layer 128, where it has not been exposed to light, while keepingit in place overlying defect-rich regions 134-i. Next, masking layer 128s removed as well. This procedure creates a planar lattice of alignedmask-layers 148-i overlying defect-rich regions 134-i.

Next, first electrode layer 104 is deposited overlying the planarlattice of aligned mask-layers 148-i.

FIG. 8F illustrates the next step, in which first electrode layer 104 ispartially removed by the lift-off of planar lattice of alignedmask-layers 148-i. This step exposes n-doped layer 108 in defect-richregions 134-i, but still leaves n-doped layer 108 covered by firstelectrode layer 104 in defect-poor regions 138-i.

FIG. 8G illustrates the formation of holes 122-i by etching, whichleaves first electrode layer 104 in place, but removes the exposedepi-layers 124 in defect-rich regions 138-i. First electrode layer 104is used as an etch mask in this step. Epi-layers 124 can be removedcompletely or partially to form holes 122-i. In some embodiments dryetching is used to make the walls of holes 122-i approximately vertical.After this step the remaining portions of first electrode layer 104 areelectrically coupled only to n-doped layer 108.

Because of the ELOG technique lattice of openings 130-i of masking layer128 are aligned with defect-rich regions 134-i. Therefore, the etchingstep of FIG. 8G substantially removes defect-rich regions 134-i,substantially leaving defect-poor regions 138-i in place. Thus, LEDscreated by the ELOG technique have low defect density, reducing thementioned detrimental effects, including poor reliability, reducedefficiency, and diminished brightness.

In the next step second electrode layer 120 is formed over a region ofp-doped layer 116 that is displaced from the photonic crystal structure,making its fabrication easier.

In substrate-emitting PXLEDs bonding layer 121 is substantiallytransparent, formed from, for example, indium tin oxide (“ITO”). Secondsubstrate 142 is also substantially transparent, formed from, forexample, sapphire, silicon carbide or glass. First electrode layer 104is substantially reflective or opaque, formed from, for example, Ag, Alor Au.

In top-emitting PXLEDs, at least one of bonding layer 121 and secondsubstrate 142 are substantially reflective or opaque. Bonding layer 121or second substrate 142 can be made reflective, for example, by forminga substantially reflective overlying layer.

In some embodiments the order of deposition of n-doped layer and p-dopedlayer is reversed, thus layer 108 is p-doped, while layer 116 isn-doped.

FIGS. 9A-F illustrate a related method of fabricating PXLEDs. The steps,shown in FIGS. 9A-D are the same as in FIGS. 8A-D.

FIG. 9E illustrates the next step, in which masking layer 128 is used asan etch layer. Therefore, defect-rich regions 134-i are partiallyremoved in this step, creating holes 122-i where lattice of openings130-i of masking layer 128 were originally located. After the formationof holes 122-i masking layer 128 is removed. After the formation ofholes 122-i, the total thickness of epi-layers 124 can be optimized byfurther etching or other techniques.

FIG. 9F illustrates the next step, in which holes 122-i are filled upwith a non-conducting material 143 to make the upper surface of thedevice approximately flat. Non-conducting material can be, for example,a spin-on-glass (“SOG”). Then first electrode layer 104 is depositedover the approximately flat upper surface, formed by n-doped layer 108and non-conducting material 143. By this architecture first electrodelayer 104 is electrically coupled only to n-doped layer 108. Next,second electrode layer 120 is formed over a region of p-doped layer 116that is displaced from the photonic crystal structure, making itsfabrication easier. In analogy to the embodiment of FIGS. 8A-G, thePXLED fabricated by the method of FIGS. 9A-F can be a substrate-emittingor a top-emitting device.

Since in the above-described embodiments first electrode layer 104 andsecond electrode layer 120 are formed at horizontally removed areas, theflow of the currents includes substantially horizontal pathways.

In some embodiments second substrate 142 is a good conductor, thussecond electrode layer 120 can be deposited on epi-layers 124 directly,or bonding layer 121 can act as a second electrode layer. In theseembodiments the pathways of the currents are substantially verticalacross epi-layers 124.

FIGS. 10A-E illustrate another method of fabricating PXLEDs. This methodagain utilizes the epitaxial lateral overgrowth, or ELOG, technique thatcan be useful, for example, for III-Nitride based semiconductorstructures, such as GaN-based LEDs.

FIG. 10A illustrates the step of forming masking layer 128 on hostsubstrate 102. Lattice of openings 130-i can be formed in masking layer128 by high resolution lithographic techniques, such as electron beamlithography, nano-imprint lithography, deep X-ray lithography,interferometric lithography, hot embossing or microcontact printing.

FIG. 10B illustrates the step of forming n-doped layer 108 overlyingfirst substrate 102 and masking layer 128. Active layer 112 is formedoverlying n-doped layer 108, and p-doped layer 116 is formed overlyingactive layer 112. A feature of the ELOG technique is that n-doped layer108 primarily grows starting from first substrate 102 through lattice ofopenings 130-i. Thus, the growing n-doped layer 108 spreads outlaterally into regions 138-i, rather than grow straight up from maskinglayer 128.

The defects are typically nucleated by first substrate 102, and hencewill originate primarily in lattice of openings 130-i. As the growth ofn-doped layer 108 spreads out into regions 138-i, the defects anddislocations tend to turn out and annihilate one another in theovergrown region. Therefore, the defect concentration will be high indefect-rich regions 134-i directly above lattice of openings 130-i,whereas the defect concentration will be low in defect-poor regions138-i, between lattice of openings 130-i.

FIG. 10C illustrates the step of aligning the openings of a maskinglayer with defect-rich regions 134-i. The method utilizes the Talboteffect, described by W. H. F. Talbot in “Facts relating to opticalscience, No. IV,” Philosophical Magazine, vol. 9, p. 401-407, publishedby Taylor and Francis in 1836.

According to the Talbot effect, periodic structures of period length aform images of themselves at integer multiples of the distance D=2 a²/λthrough Fresnel diffraction, when illuminated by a coherent light with aplanar wave front, having a wavelength λ in the material.

In order to make use of the Talbot effect, the thickness of epi-layers124 is chosen to be D, or an integer multiple of D. Further, substrate102 is formed from a substantially transparent material, and maskinglayer is formed from a substantially nontransparent material. Also, aphotosensitive layer 149 is deposited overlying p-doped layer 116. TheTalbot effect is utilized by perpendicularly shining a light with aplanar wave front at the side of substrate 102 opposite to epi-layers124. Only that part of the light will enter epi-layers 124, which wasincident at lattice of openings 130-i. The light, propagating throughlattice of openings 130-i, creates the image of the lattice of openings130-i at a distance D because of the Talbot effect. Thus, photosensitivelayer 149 will be exposed to the image of lattice of openings 130-i. Theexposed regions of photosensitive layer 149 are removed in a subsequentstep to create aligned openings 150-i. The Talbot effect can be achievedin the present embodiment, for example, by using a near collimated lightsource.

FIG. 10D illustrates the step of using aligned openings for formingholes 122-i. For example, dry etching can be used to ensure that thewalls of holes 122-i will be approximately vertical. By virtue of theTalbot effect aligned openings 150-i are aligned with defect-richregions 134-i. Therefore, the etching step removed defect-rich regions134-i, so that the remaining epi-layers 124 substantially consist ofdefect-poor regions 138-i. Therefore, PXLEDs created by this techniquewill have low defect density. After the etching step, photosensitivelayer 149 is removed.

FIG. 10E illustrates the step of forming first electrode layer 104overlying n-doped layer 108, and second electrode layer 120 overlyingp-doped layer 116. First electrode layer 104 is formed over a region ofn-doped layer 108 that is displaced from the photonic crystal structure,making its fabrication easier. Second electrode layer 120 is formed canbe formed, for example, by deposition from an angle. This techniqueminimizes the deposition of contact materials inside holes 122-i.

FIGS. 1A-E illustrate a method, related to the method of FIGS. 10A-E.The steps of FIGS. 11A and 11B are the same as the steps of FIGS. 10Aand 10B.

FIG. 11C illustrates a different way of utilizing the Talbot effect. Thethickness of epi-layers 124 is chosen to be D, or an integer multiple ofD. Further, substrate 102 is formed from a substantially transparentmaterial, and masking layer 128 is formed from a substantiallynon-transparent material. Also, a negative photosensitive layer isdeposited overlying p-doped layer 116. The Talbot effect is utilized byperpendicularly shining a light with a planar wave front at the side ofsubstrate 102 opposite to epi-layers 124. Only that part of the lightwill enter epi-layers 124, which was incident at lattice of openings130-i. The light, propagating through lattice of openings 130-i, createsthe image of the lattice of openings 130-i at a distance D because ofthe Talbot effect. Thus, the photosensitive layer will be exposed to theimage of lattice of openings 130-i. The non-exposed regions of thephotosensitive layer are removed in a subsequent step to create alignedmask-layers 148-i. The Talbot effect can be achieved in the presentembodiment, for example, by using a near collimated light source.

Next, second electrode layer 120 is formed overlying p-doped layer 116and the photosensitive layer.

FIG. 11D illustrates the step of forming aligned openings 150-i by alift-off technique. Aligned mask-layers 148-i are removed, together withthe corresponding portions of second electrode layer 120 to exposep-doped layer 116 in defect-rich regions 134-i. By virtue of the Talboteffect, aligned mask-layers 148-i are aligned with defect-rich regions134-i. Therefore, aligned openings 150-i will be aligned withdefect-rich regions 134-i.

FIG. 11E illustrates the next step, where defect-rich regions 134-i areat least partially removed, while keeping second electrode layer 120intact. Defect-rich regions 134-i are removed sufficiently deeply toreach n-doped layer 108. This step forms holes 122-i. Finally, firstelectrode layer 104 is formed is formed over a region of n-doped layer108 that is displaced from the photonic crystal structure, making itsfabrication easier.

This method removes substantially defect rich regions 134-i, so that theremaining epi-layers 124 comprise substantially defect-poor regions138-i. Thus, PXLEDs fabricated by this method have low defect density,reducing the mentioned detrimental effects, including poor reliability,reduced efficiency, and diminished brightness.

In substrate-emitting PXLEDs host substrate 102 is formed from asubstantially transparent material, for example, sapphire, siliconcarbide or glass and second electrode layer 120 is formed from asubstantially reflective or opaque material, for example, Ag, Al or Au.In top-emitting PXLEDs host substrate 102 is substantially reflective oropaque, for example, metallized sapphire. In some embodiments, secondelectrode layer 120 is formed from a substantially transparent material,for example, ITO, or a thin metal layer.

FIGS. 12A-E illustrate a method, related to the method of FIGS. 11A-E.The steps of FIGS. 12A and 12B are the same as the steps of FIGS. 11Aand 11B.

FIG. 12C illustrates an alternative utilization of the Talbot effect. Inthis method a photo resist is deposited as a photosensitive layer onp-doped layer 116. The photo resist is exposed using the Talbot effect.In a subsequent step the exposed portions of the photosensitive layer isremoved from defect-rich regions 134-i to create a photosensitive layer149 with aligned openings 150-i.

FIG. 12D illustrates the next step, in which defect-rich regions 134-iare at least partially removed to form holes 122-i, and thenphotosensitive layer 149 is removed. Again, defect-rich regions 134-iare removed sufficiently deeply to reach n-doped layer 108.

FIG. 12E illustrates the next step, in which holes 122-i are filled upwith a non-conducting material 143 to make the upper surface of thedevice approximately flat. Non-conducting material can be, for example,a spin-on-glass (“SOG”). Then second electrode layer 120 is depositedover the approximately flat upper surface, formed by p-doped layer 116and non-conducting material 143. By this architecture second electrodelayer 120 is electrically coupled only to p-doped layer 116. Next, firstelectrode layer 104 is formed over a region of n-doped layer 108 that isdisplaced from the photonic crystal structure, making its fabricationeasier. In analogy to the embodiment of FIGS. 11A-E, the PXLEDfabricated by the method of FIGS. 12A-E can be a substrate-emitting or atop-emitting device.

Since in the above-described embodiments first electrode layer 104 andsecond electrode layer 120 are formed at horizontally removed areas, theflow of the currents includes substantially horizontal pathways.

In some embodiments host substrate 102 is a good conductor, thus firstelectrode layer 104 can be deposited on host substrate 102 before theformation of epi-layers 124. In these embodiments the pathways of thecurrents are substantially vertical across epi-layers 124.

FIG. 13 illustrates an embodiment of PXLED 100 in a high-power package.For example, PXLEDs with an area of 1 mm² or greater can be packaged inhigh-power packages. The high-power package includes a heat sink 204,formed from a low thermal resistance material. Heat sink 204 also servesas a reflector cup, reflecting the light emitted from LED 200 towardsthe base of the package. A further function of heat sink 204 is toaccommodate and compensate the effects of the thermal expansion of thepackaged LED's components. LED 200 is attached to heat sink 204 withsolder or die-attach-epoxy. LED 200 is electrically coupled to innerleads 208 by wirebonds 212. In some embodiments LEDs with inverted, orflip-chip, design are electrically coupled to inner leads 208 bysolderballs or solderbars. Inner leads 208 are electrically coupled toouter leads 216. Inner leads 208, outer leads 216, and wirebonds areformed from suitably chosen metals. LED 200 is encapsulated into atransparent housing that includes an epoxy dome lens 220 for enhancedlight extraction. A soft gel 224 with high refractive index is disposedbetween LED 200 and epoxy dome lens 220 to enhance light extraction. Thepackaged LED is structurally supported by a support frame 228. Inembodiments, where the extraction efficiency of the LED is between about50% and 100% in air, the inclusion of lens 220 and soft gel 224 is notnecessary.

There are a large number of different packages the PXLEDs can be housedin. The choice of the most suitable package depends, among others, theparticular application.

The embodiments discussed above are exemplary only and are not intendedto be limiting. One skilled in the art will recognize variations fromthe embodiments described above, which are intended to be within thescope of the disclosure. As such, the invention is limited only by thefollowing claims.

1. A light emitting diode comprising: a first semiconductor layer dopedwith a first dopant, coupled to a first electrode layer; an active layeroverlying said first semiconductor layer, capable of emitting light; asecond semiconductor layer doped with a second dopant, overlying saidactive layer, said first dopant and said second dopant being of oppositetype; a second electrode layer on said second semiconductor layer; and aperiodically-arranged plurality of holes formed in the secondsemiconductor layer and extending towards the first semiconductor layer,wherein the ratio of the period of said periodic arrangement and thewavelength of said emitted light in air is greater than about 0.1 andless than about 5; a depth of at least one of the plurality of holes issuch that a thickness of said second semiconductor layer at a bottom ofsaid at least one of the plurality of the holes is less than about onewavelength of said emitted light in said second semiconductor layer; aportion of the second electrode layer is disposed in a region of thesecond semiconductor layer in which a portion of the plurality of holesare formed; and when forward biased, light is emitted from at least aportion of the active layer disposed beneath a portion of the secondelectrode.
 2. The light emitting diode of claim 1, wherein said firstdopant is n-type and said second dopant is p-type.
 3. The light emittingdiode of claim 1, wherein said first semiconductor layer overlies saidfirst electrode layer.
 4. The light emitting diode of claim 1, whereinsaid first electrode layer partially overlies said first semiconductorlayer; and said first semiconductor layer overlies a substrate with areflective surface.
 5. The light emitting diode of claim 1, wherein saidfirst electrode layer partially overlies said first semiconductor layer;said second electrode layer is reflective; and said first semiconductorlayer overlies a transparent substrate.
 6. The light emitting diode ofclaim 1, wherein a surface in one of the plurality of holes has asurface recombination velocity less than 10⁵ cm/sec.
 7. The lightemitting diode of claim 1, wherein said first semiconductor layer, saidactive layer, and said second semiconductor layer comprise a group IIIelement and a group V element.
 8. The light emitting diode of claim 1,wherein said first semiconductor layer, said active layer, and saidsecond semiconductor layer comprise GaN.
 9. The light emitting diode ofclaim 1, wherein said periodically-arranged plurality of holes isperiodic in at least one direction parallel to a plane of said secondsemiconductor layer.
 10. The light emitting diode of claim 1, whereinsaid periodic arrangement comprises a planar lattice of holes.
 11. Thelight emitting diode of claim 10, wherein said planar lattice is atriangular lattice, square lattice, or a hexagonal lattice.
 12. Thelight emitting diode of claim 10, wherein said planar lattice is ahoneycomb lattice.
 13. The light emitting diode of claim 12, whereinsaid emitted light has an intensity and a polarization and the intensityof said emitted light is substantially independent of the polarization.14. The light emitting diode of claim 1, wherein said holes are filledwith a dielectric.
 15. The light emitting diode of claim 14, whereinsaid dielectric is silicon oxide.
 16. The light emitting diode of claim1, wherein the periodically-arranged plurality of holes form a photoniccrystal having a photonic crystal band structure comprising one or morebands with edges; and an energy of said emitted light lies close to anedge of a band of the photonic crystal band structure.
 17. The lightemitting diode of claim 16, wherein the product of a rate of spontaneousemission of the light emitting diode and an efficiency of lightextraction of the light emitting diode is greater at an energy close tosaid band edge than at a plurality of energies away from said band edge.18. The light emitting diode of claim 14, wherein the dielectricconstants of said dielectric, said active layer and said secondsemiconductor layer assume values between about 1 and about 14; and saidholes occupy between about 10% and about 50% of the area of said secondsemiconductor layer.
 19. The light emitting diode of claim 1, wherein anintensity of light emitted in a direction normal to a plane of saidsecond semiconductor layer is greater than an intensity of light emittedin a direction different from the normal of the plane of said secondsemiconductor layer.
 20. The light emitting diode of claim 1, whereinsaid first semiconductor layer and said second semiconductor layer eachcomprise at least one layer of a III-nitride material; said active layercomprises InGaN; said first and second electrode layers comprise atleast one of Ag, Al, and Au; said periodically-arranged plurality ofholes is a triangular lattice of holes, wherein a diameter of said holesis between about 0.3 a and about 0.72 a, wherein a is the period of theperiodic arrangement; a depth of said holes is between about 0.375 a andabout 2 a; and said first and second semiconductor layers together forman epi-layer, having a thickness between about 0.375 a and about 2 a.21. The light emitting diode of claim 1, wherein said firstsemiconductor layer and said second semiconductor layer each comprise atleast one layer of a III-nitride material; said active layer comprisesInGaN; said first and second electrode layers comprise at least one ofAg, Al, and Au; said periodically-arranged plurality of holes is atriangular lattice of holes, wherein a diameter of said holes is betweenabout 0.3 a and about 0.72 a, wherein a is the period of the periodicarrangement; a depth of said holes is greater than about 2 a; and saidfirst and second semiconductor layers together have a thickness greaterthan about 4 a.
 22. The light emitting diode of claim 1, wherein saidlight emitting diode is disposed in a package, the package comprising: asupport frame; a heat sink disposed within said support frame forextracting heat from said light emitting diode, wherein said lightemitting diode is disposed over said heat sink; a plurality of leads,electrically coupled to said light emitting diode; and a transparenthousing overlying the light emitting diode.
 23. A light emitting diodecomprising: a first semiconductor layer doped with a first dopant,coupled to a first electrode layer; an active layer overlying said firstsemiconductor layer, capable of emitting light; a second semiconductorlayer doped with a second dopant overlying said active layer, said firstand second dopants being of opposite type; a second electrode layer onsaid second semiconductor layer; and a periodically-arranged pluralityof holes formed in the second semiconductor layer and extending towardsthe first semiconductor layer, wherein: the ratio of the period of saidperiodic arrangement and the wavelength of said emitted light in air isgreater than about 0.1 and less than about 5; a depth of at least one ofthe plurality of holes is such that the thickness of said secondsemiconductor layer at a bottom of said at least one of the plurality ofholes is less than about one wavelength of said emitted light in saidsecond semiconductor layer; a portion of the second electrode layer isdisposed in a region of the second semiconductor layer in which aportion of the plurality of holes are formed; when forward biased, lightis emitted from at least a portion of the active layer disposed beneatha portion of the second electrode; and at least one of said firstsemiconductor layer, said active layer, and said second semiconductorlayer composes a group III element and nitrogen.
 24. The light emittingdiode of claim 23, wherein said group III element is Gallium.
 25. Thelight emitting diode of claim 23, wherein a surface in one of theplurality of holes has a surface recombination velocity less than 10⁵cm/sec.
 26. The light emitting diode of claim 23, wherein said firstdopant is n-type and said second dopant is p-type.
 27. The lightemitting diode of claim 23 wherein said first semiconductor layeroverlies said first electrode layer.
 28. The light emitting diode ofclaim 23, wherein said first electrode layer partially overlies saidfirst semiconductor layer; and said first semiconductor layer overlies asubstrate with a reflective surface.
 29. The light emitting diode ofclaim 23, wherein said first electrode layer partially overlies saidfirst semiconductor layer; said second electrode layer is reflective;and said first semiconductor layer overlies a transparent substrate. 30.The light emitting diode of claim 23, wherein said periodically-arrangedplurality of holes is periodic in at least one direction parallel to aplane of said second semiconductor layer.
 31. The light emitting diodeof claim 23, wherein said periodic arrangement comprises a planarlattice of holes.
 32. The light emitting diode of claim 31, wherein saidplanar lattice is a triangular lattice, a square lattice, or a hexagonallattice.
 33. The light emitting diode of claim 31, wherein said planarlattice is a honeycomb lattice.
 34. The light emitting diode of claim33, wherein said emitted light has an intensity and a polarization andthe intensity of said emitted light is independent of the polarization.35. The light emitting diode of claim 23, wherein said holes are filledwith a dielectric.
 36. The light emitting diode of claim 35, whereinsaid dielectric is silicon oxide.
 37. The light emitting diode of claim23, wherein the periodically-arranged plurality of holes form a photoniccrystal having a photonic crystal band structure comprising one or morebands with edges; and an energy of said emitted light lies close to anedge of a band of the photonic crystal band structure.
 38. The lightemitting diode of claim 37, wherein the product of a rate of spontaneousemission of the light emitting diode and an efficiency of lightextraction of the light emitting diode is greater at an energy close tosaid band edge than at a plurality of energies away from said band edge.39. The light emitting diode of claim 35, wherein dielectric constantsof said dielectric, said first semiconductor layer, and said secondsemiconductor layer assume values between about 1 and about 16; and saidholes occupy between about 10% and about 50% of the area of said secondsemiconductor layer.
 40. The light emitting diode claim of 23, whereinan intensity of light emitted in a direction normal to a plane of saidsecond semiconductor layer is greater than an intensity of light emittedin a direction different from a normal of the plane of said secondsemiconductor layer.
 41. The light emitting diode of claim 23, whereinsaid first semiconductor layer and said second semiconductor layer eachcomprise at least one layer of a III-nitride material; said active layercomprises InGaN; said periodically-arranged plurality of holes is atriangular lattice of holes, wherein a diameter of said holes is betweenabout 0.3 a and about 0.72 a, wherein a is the period of theperiodically-arranged plurality of holes; a depth of said holes isbetween about 0.375 a and about 2 a; and said first and secondsemiconductor layers together form an epi-layer, having a thicknessbetween about 0.375 a and about 2 a.
 42. The light emitting diode ofclaim 23, wherein said first semiconductor layer and said secondsemiconductor layer each comprise at least one layer of a III-nitridematerial; said active layer comprises InGaN; said periodically-arrangedplurality of holes is a triangular lattice of holes, wherein a diameterof said holes is between about 0.3 a and about 0.72 a, wherein a is theperiod of the periodically-arranged plurality of holes; a depth of saidholes is greater than about 2 a; and said first and second semiconductorlayers together have a thickness greater than about 4 a.
 43. The lightemitting diode of claim 1, wherein at least one of the holes extendsthrough the second semiconductor layer and into the active region. 44.The light emitting diode of claim 1, wherein at least one of the holesextends through the second semiconductor layer, through the activeregion, and into the first semiconductor layer.
 45. The light emittingdiode of claim 1, wherein the periodically arranged plurality of holescomprises parallel grooves.
 46. The light emitting diode of claim 23,wherein at least one of the holes extends through the secondsemiconductor layer and into the active region.
 47. The light emittingdiode of claim 23, wherein at least one of the holes extends through thesecond semiconductor layer, through the active region, and into thefirst semiconductor layer.
 48. The light emitting diode of claim 23,wherein the periodically arranged plurality of holes comprises parallelgrooves.
 49. The light emitting diode of claim 20, wherein a diameter ofsaid holes is about 0.72 a and said holes are filled with air.
 50. Thelight emitting diode of claim 20, wherein said light, emitted by saidactive layer, has a frequency between about 0.66(c/a) and about0.75(c/a), wherein c is the speed of light in air.
 51. The lightemitting diode of claim 21, wherein said holes are filled with air. 52.The light emitting diode of claim 21, wherein said light, emitted bysaid active layer has a frequency in one of the ranges of about 0.2(c/a)to about 0.4(c/a) and about 0.5(c/a) to about 0.8(c/a), wherein c is thespeed of light in air.
 53. The light emitting diode of claim 41, whereina diameter of said holes is about 0.72 a and said holes are filled withair.
 54. The light emitting diode of claim 41, wherein said light,emitted by said active layer, has a frequency between about 0.66(c/a)and about 0.75(c/a), wherein c is the speed of light in air.
 55. Thelight emitting diode of claim 42, wherein said holes are filled withair.
 56. The light emitting diode of claim 42, wherein said light,emitted by said active layer has a frequency in one of the ranges ofabout 0.2(c/a) to about 0.4(c/a) and about 0.5(c/a) to about 0.8(c/a),wherein c is the speed of light in air.